Cellulose ester film has been employed as a photographic negative film support, and in polarizing plates as a film which protects a polarizer employed in liquid crystal displays, due to its high transparency, low birefringence, and ease of adhesion to a polarizer.
In recent years, the production amount of liquid crystal displays has markedly increased due to the only limited depth and light weight, and the demand is increasing. Further, television sets, which employ a liquid crystal display, exhibit features such as thinness and light weight. Thereby, large-sized television sets, production of which had been difficult by employing a cathode ray tube, have been produced. Along with that trend, demand for polarizers and polarizer protecting films has been increasing.
So far, these cellulose ester films have been produced mainly employing a solution-casting method. The solution-casting method, as descried herein, refers to a film forming method in which a solution prepared by dissolving cellulose ester in a solvent is cast to form a film and the solvent is evaporated to obtain a dried film. The film which is cast employing the solution-casting method exhibits high flatness, whereby by employing the resulting film, it is possible to produce uniform and high image quality liquid crystal displays.
However, an inherent problem of the solution-casting method is the necessity of a large volume of organic solvents followed by a high environmental load. The cellulose ester film is cast employing a halogen-containing solvent which result in a high environmental load, due to the high solubility of cellulose ester in the halogen-containing solvent. Consequently, it has particularly demanded to reduce the amount of used solvents, whereby it has been difficult to increase the production of cellulose ester film employing the solution-casting method.
Accordingly, in recent years, challenges have been conducted in which cellulose ester is subjected to melt-casting for the use of silver salt photography (Patent Document 1) and as a polarizer protective film (Patent Document 2). However, cellulose ester is a polymer which exhibits a very high viscosity when melted and also exhibits a very high glass transition point. As a result, when cellulose ester is melted and extruded from a die to cast onto a cooling drum or belt, leveling of the film is not easy, since solidification occurs in a relatively short time after extrusion. Accordingly, the major problem has been that the flatness of the resulting film is lower than that of a solution-cast film.
In order to lower the melt viscosity and glass transition point of organic polymers such as cellulose ester, it is known that addition of plasticizers is effective.
In above Patent Documents 1 and 2, employed are phosphoric acid plasticizers such as triphenyl phosphate or phenylenebisdiphenyl phosphate.
However, as the result of examination conducted by the present inventors, it was found that, in these phosphoric acid plasticizers, phosphoric acid esters undergo decomposition due to moisture absorption or heat, resulting in generating phosphoric acid which deteriorates the cellulose ester and causes coloring of the film.
In Patent Documents 3, 4 and 5, ethylene glycol based plasticizers or polyalcohol esters which are esters of polyalcohols of trihydric or more with carboxylic acids have been disclosed as plasticizers employable in solution casting.
These plasticizers are favorable materials for cellulose ester since these materials exhibit high chemical stability and generate no strong acid which causes deterioration of cellulose ester even when hydrolyzed. However, since most of these materials are alkyl esters, effects of reducing moisture permeability is not fully enough. In order to obtain sufficient moisture permeability, the adding amount of a plasticizer may merely be increased, however, such simple increase of the plasticizer may result in bleeding out of the plasticizer on the surface of the cellulose ester film or in loss of transparency of the film.
The use of low molecular weight plasticizers such as triphenyl phosphate or dioctyl adipate may result in loss of flatness of the film due to deposition of vaporized plasticizer on the die when the film is melt-cast. Accordingly, a low volatile plasticizer is desired.
As examples of low volatile plasticizers, polyester plasticizers or polyurethane plasticizers have been proposed in Patent Documents 6-9, however, the plasticizers so far proposed tend to exhibit low compatibility with cellulose ester resulting in loss of transparency of the film.
Patent Document 1 teaches a stretching ratio of 2 to 4 to obtain a melt-cast cellulose ester film, however, a cellulose ester film is inherently a rather difficult polymer film to stretch, and as described in the document, it can be stretched at most 20 to 40% at ambient temperature. Accordingly, even at a higher temperature, it has been rather difficult to stretch a cellulose ester film in a ratio of 2 to 4 with sufficient manufacturing efficiency.
(Patent Document 1) Published Japanese Translation of International Publication No. 6-501040
(Patent Document 2) Japanese Patent Application Publication Open to Public Inspection (hereinafter referred to as JP-A) No. 2000-352620
(Patent Document 3) JP-A No. 11-246704
(Patent Document 4) JP-A No. 2000-63560
(Patent Document 5) JP-A No. 11-124445
(Patent Document 6) JP-A No. 2002-22956
(Patent Document 7) JP-A No. 2002-267846
(Patent Document 8) JP-A No. 2004-175971
(Patent Document 9) JP-A No. 2004-175972